Early defoliation is a sign of stress. Some trees are turning now but they are mostly Ashes, a tree with many stresses and Maples. Sometimes it is a sign of girdling roots. The sap could be something or nothing. If it is oozing from cankers, it could be a sign of disease.

I suggest having a certified arborist check the tree. If that is not a possibility, I might be able to help if you send me some digitial photos. You can e-mail them to me at: chris@treecareadvisor.com